Exhaust Gas Control Apparatus for Internal Combustion Engine

ABSTRACT

When the temperature of exhaust gas control means ( 3 ), which is provided in an exhaust passage ( 2 ) of an internal combustion engine ( 1 ) and which has oxidation function, is increased to a target temperature in order to recover an exhaust gas control ability of the exhaust gas control means ( 3 ), the method of increasing the temperature of the exhaust gas control means ( 3 ) is switched between a method in which the temperature of the exhaust gas control means ( 3 ) is increased by controlling an amount of fuel supplied to the exhaust gas control means ( 3 ) and a method in which the fuel supply to the exhaust gas control means ( 3 ) is prohibited and the temperature of the exhaust gas control means ( 3 ) is increased by controlling an amount of air taken in the internal combustion engine ( 1 ), based on an operation state of the internal combustion engine ( 1 ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an exhaust gas control apparatus for an internal combustion engine, which includes exhaust gas control means that is provided in an exhaust passage of an internal combustion engine and that has oxidation function.

2. Description of the Related Art

In an exhaust gas control apparatus for an internal combustion engine, a particulate filter (hereinafter, referred to as a “filter”) which traps particulate matter (hereinafter, referred to as “PM”) in exhaust gas, a NOx storage reduction catalyst, or the like may be provided in an exhaust passage of an internal combustion engine. In the case where the filter is provided in the exhaust passage, the temperature of the filter needs to be increased when PM accumulated in the filter is removed. In the case where the NOx storage reduction catalyst is provided in the exhaust passage, the temperature of the NOx storage reduction catalyst needs to be increased when SOx stored in the NOx storage reduction catalyst is reduced.

There is a known technology related to an exhaust gas control apparatus for an internal combustion engine, in which a filter serving as exhaust gas control means is provided in an exhaust passage. According to the technology, when PM accumulated in the filter is removed by increasing the temperature of the filter in order to recover the exhaust gas control ability of the filter, one of the following methods is selected based on an operation state of the internal combustion engine, and the temperature of the filter is increased by the selected method. The methods are (1) a method in which the temperature of the exhaust gas is increased by performing a normal operation in a high load operation region, (2) a method in which fuel injection timing is retarded, (3) a method in which sub-fuel injection is performed during power stroke, and EGR gas is introduced, and (4) a method in which sub-fuel injection is performed during power stroke, and a flow rate of intake air/exhaust gas is decreased. Such a technology is disclosed in, for example, Japanese Patent Application Publication No. JP (A) 2000-161044.

Also, there is a technology related to an exhaust gas control apparatus for an internal combustion engine, in which exhaust gas control means provided in an exhaust passage has oxidation function. According to the technology, fuel is supplied to the exhaust gas control means from a position upstream of the exhaust gas control means, in order to increase the temperature of the exhaust gas control means. Such a technology is disclosed in, for example, Japanese Patent Application Publication No. JP (A) 2002-285896, Japanese Patent Application Publication No. JP (A) 2002-235589, and Japanese Patent Application Publication No. JP (A) 05-106518.

In the exhaust gas control apparatus for an internal combustion engine including the exhaust gas control means that is provided in the exhaust passage of the internal combustion engine and that has oxidation function, when the temperature of the exhaust gas control means is increased in order to recover the exhaust gas control ability of the exhaust gas control means, for example, fuel is supplied to the exhaust gas control means from a position upstream of the exhaust gas control means by performing the sub-fuel injection that is performed after the main fuel injection in the internal combustion engine, and supplying fuel to the exhaust passage located upstream of the exhaust gas control means. In this case, the temperature of the exhaust gas control means is increased by oxidation heat generated due to oxidation of the fuel that occurs in the exhaust gas control means.

However, it is sometimes difficult, due to the operation state of the internal combustion engine, to supply the exhaust gas control means with a sufficient amount of fuel that is required for increasing the temperature of the exhaust gas control means to a target temperature at which the exhaust gas control ability of the exhaust gas control means can be recovered. For example, as a load placed on the internal combustion engine (hereinafter, referred to as an “engine load of the internal combustion engine”) becomes higher, the temperature in a cylinder and the exhaust gas temperature increase. Accordingly, there is a possibility that the fuel injected by the sub-fuel injection or the fuel supplied to the exhaust passage is burned in the cylinder or the exhaust passage, and therefore, a sufficient amount of fuel is not supplied to the exhaust gas control means.

SUMMARY OF THE INVENTION

The invention is made in light of the above-mentioned circumstances. It is, therefore, an object of the invention to provide a technology that makes it possible to increase a temperature of exhaust gas control means provided in an exhaust passage of an internal combustion engine to a target temperature in a broader range of an operation state, thereby recovering an exhaust gas control ability of the exhaust gas control means more appropriately, in an exhaust gas control apparatus for an internal combustion engine.

According to the invention, when the temperature of exhaust gas control means, which is provided in an exhaust passage of an internal combustion engine and which has oxidation function, is increased to a target temperature in order to recover an exhaust gas control ability of the exhaust gas control means, the method of increasing the temperature of the exhaust gas control means is switched between a method in which the temperature of the exhaust gas control means is increased by controlling an amount of fuel supplied to the exhaust gas control means and a method in which the fuel supply to the exhaust gas control means is prohibited and the temperature of the exhaust gas control means is increased by controlling an amount of air taken in the internal combustion engine, based on an operation state of the internal combustion engine.

More particularly, according to the invention, there is provided an exhaust gas control apparatus for an internal combustion engine including exhaust gas control means which is provided in an exhaust passage of an internal combustion engine, and which has oxidation function; fuel supply means for supplying fuel to the exhaust gas control means from a position upstream of the exhaust gas control means; and intake air amount control means for controlling an amount of air taken in the internal combustion engine, characterized in that when a temperature increasing condition for increasing a temperature of the exhaust gas control means to a target temperature in order to recover an exhaust gas control ability of the exhaust gas control means has been satisfied, if an engine load of the internal combustion engine is lower than a predetermined engine load, the temperature of the exhaust gas control means is increased to or maintained at the target temperature by controlling an amount of fuel supplied from the fuel supply means to the exhaust gas control means, and the intake air amount control means controls the amount of air taken in the internal combustion engine to an intake air amount that is set based on an engine rotational speed of the internal combustion engine and the engine load of the internal combustion engine; and when the temperature increasing condition has been satisfied, if the engine load of the internal combustion engine is equal to or higher than the predetermined engine load, fuel supply from the fuel supply means is prohibited, and the intake air amount control means controls the amount of air taken in the internal combustion engine, whereby the temperature of the exhaust gas control means is increased to or maintained at the target temperature.

Examples of the exhaust gas control means according to the invention include a filter which supports an oxidation catalyst or a NOx storage reduction catalyst, and a NOx storage reduction catalyst.

The fuel supply means according to the invention may perform sub-fuel injection in a cylinder of the internal combustion engine during a power stroke or an exhaust stroke after main fuel injection is performed, thereby supplying fuel to the exhaust gas control means. Also, the fuel supply means may supply fuel to the exhaust passage located upstream of the exhaust gas control means by using a fuel supply valve, thereby supplying the fuel to the exhaust gas control means.

The temperature increasing condition is changed depending on the types of the exhaust gas control means. For example, when the exhaust gas control means is a filter, the temperature increasing condition may be a condition in which removal of the PM accumulated in the filter is performed. When the exhaust gas control means is a NOx storage reduction catalyst, the temperature increasing condition may be a condition in which reduction of the SOx stored in the NOx storage reduction catalyst is performed. Also, the target temperature is changed depending on the purposes of increasing the temperature.

In the invention, when the temperature increasing condition for increasing the temperature of the exhaust gas control means to the target temperature has been satisfied, if the engine load of the internal combustion engine is lower than the predetermined engine load, the temperature of the exhaust gas control means is increased to or maintained at the target temperature by controlling the amount of fuel supplied from the fuel supply means to the exhaust gas control means. At this time, the intake air amount control means controls the amount of air taken in the internal combustion engine to an intake air amount that is set based on the engine rotational speed of the internal combustion engine and the engine load of the internal combustion engine.

However, as described above, as the engine load of the internal combustion engine increases, the temperature in the cylinder and the exhaust gas temperature increase. Accordingly, there is a possibility that the fuel supplied by the fuel supply means from a position upstream of the exhaust gas control means is burned in the cylinder or the exhaust passage, in a region where the engine load is high.

Therefore, in the invention, when the temperature increasing condition for increasing the temperature of the exhaust gas control means to the target temperature has been satisfied, if the engine load of the internal combustion engine is equal to or higher than the predetermined engine load, the fuel supply from the fuel supply means is prohibited. Then, the intake air amount control means controls the amount of air taken in the internal combustion engine, whereby the temperature of the exhaust gas control means is increased to or maintained at the target temperature.

The predetermined engine load may be a threshold value of the engine load, at which the temperature in the cylinder or the exhaust gas temperature becomes high and the fuel supplied from the fuel supply means is burned in the cylinder or the exhaust passage.

Even when fuel is not supplied to the exhaust gas control means, if the flow rate of the exhaust gas is decreased by decreasing the intake air amount, the temperature of the exhaust gas can be increased, and therefore, the temperature of the exhaust gas control means can be increased. When the engine load of the internal combustion engine is relatively high, the temperature of the exhaust gas control means can be increased to the target temperature only by controlling the intake air amount. When the engine load of the internal combustion engine is equal to or higher than the predetermined engine load, prohibition of the fuel supply makes it possible to suppress an excessive increase in the temperature of the exhaust gas, which is caused due to burning of the fuel in the cylinder or the exhaust passage.

Therefore, according to the invention, the temperature of the exhaust gas control means can be increased to or maintained at the target temperature regardless of the engine load of the internal combustion engine. Namely, the temperature of the exhaust gas control means can be increased to the target temperature in a broader range of operation state. Accordingly, the exhaust gas control ability of the exhaust gas control means can be recovered more appropriately.

When fuel is supplied to the exhaust gas control means from a position upstream of the exhaust gas control means by performing the sub-fuel injection in the cylinder of the internal combustion engine during a power stroke or an exhaust stroke after the main fuel injection is performed, as the engine rotational speed of the internal combustion engine becomes higher, the time required for one combustion cycle becomes shorter. Therefore, the number of times of the sub-fuel injection that can be performed during one combustion cycle decreases. Accordingly, in the region where the internal combustion engine is operated at a high engine rotational speed, it may be difficult to supply a sufficient amount of fuel required for increasing the temperature of the exhaust gas control means to the target temperature by performing the sub-fuel injection.

Therefore, in the invention, in the case where the fuel supply means performs sub-fuel injection in a cylinder of the internal combustion engine during the power stroke or the exhaust stroke after main fuel injection is performed, thereby supplying fuel to the exhaust gas control means from a position upstream of the exhaust gas control means, even if the engine load of the internal combustion engine is lower than the predetermined engine load when the temperature increasing condition has been satisfied, if the engine rotational speed of the internal combustion engine is equal to or higher than the predetermined engine rotational speed, fuel supply from the fuel supply means is prohibited. The intake air amount control means may control the amount of air taken in the internal combustion engine, whereby the temperature of the exhaust gas control means is increased to or maintained at the target temperature.

The predetermined engine rotational speed may be a threshold value of the engine rotational speed at which it becomes difficult to perform the sub-fuel injection number of times that makes it possible to supply the sufficient amount of fuel required for increasing the temperature of the exhaust gas control means to the target temperature.

According to the invention, when fuel supply to the exhaust gas control means is performed by the sub-fuel injection, even if the engine rotational speed of the internal combustion engine is high, the temperature of the exhaust control means can be increased to the target temperature in the broader range of the operation state. Accordingly, the exhaust gas control ability of the exhaust gas control means can be recovered more appropriately.

In the internal combustion engine, as the engine rotational speed increases, the friction that occurs between an inner surface of the cylinder and a piston increases. Accordingly, even when the engine rotational speed is high, the torque substantially equal to that when the engine rotational speed is low is generated. Accordingly, if the engine load is at substantially the same value, as the engine rotational speed increases, the amount of fuel injected by the main fuel injection is increased. However, when such control of the fuel injection amount is performed, there is a possibility that, as the engine rotational speed increases, the temperature of the cylinder and the exhaust gas temperature increase by an amount corresponding to an amount of increase in the fuel injected by the main fuel injection. As a result, the fuel supplied from the fuel supply means may be burned more easily in the cylinder or the exhaust passage, as the engine rotational speed increases.

Accordingly, in the invention, as the engine rotational speed of the internal combustion engine that is obtained when the temperature increasing condition has been satisfied increases, the predetermined engine load may be set to a lower value.

Setting the predetermined engine load in this manner makes it possible to suppress an increase in the temperature of the exhaust gas control means due to the fuel supply performed by the fuel supply means when the fuel is burned easily in the cylinder or the exhaust passage. Namely, the temperature of the exhaust gas control means can be increased to the target temperature in the broader range of the operation state.

In the invention, there may be further provided learning means for learning a relationship between the amount of air taken in the internal combustion engine and the temperature of the exhaust gas control means in a state where the fuel supply from the fuel Supply means is not performed. In a case where the temperature increasing condition has been satisfied, when the operation state of the internal combustion engine is shifted from an operation region (hereinafter, referred to as a “fuel amount control region”) where the amount of fuel supplied from the fuel supply means to the exhaust gas control means is controlled, whereby the temperature of the exhaust gas control means is increased to the target temperature, to an operation region (hereinafter, referred to as a “intake air amount control region”) where the intake air amount control means controls the amount of air taken in the internal combustion engine, whereby the temperature of the exhaust gas control means is increased to the target temperature, the relationship between the amount of air taken in the internal combustion engine and the temperature of the exhaust gas control means may be learned by the learning means after the fuel supply from the fuel supply means is stopped and the temperature of the exhaust gas control means is brought to a steady state. The amount of air taken in the internal combustion engine may be controlled by the intake air amount control means based on the relationship between the amount of air taken in the internal combustion engine and the temperature of the exhaust gas control means that is obtained by learning performed by the learning means such that the temperature of the exhaust gas control means is controlled to the target temperature.

When the temperature increasing condition has been satisfied, if the operation state of the internal combustion engine is shifted from the fuel amount control region to the intake air amount control region, the fuel supply from the fuel supply means to the exhaust gas control means is stopped. Accordingly, the temperature of the exhaust gas control means starts to be decreased. If the learning means learns the relationship between the amount of air taken in the internal combustion engine and the temperature of the exhaust gas control means while the temperature of the exhaust gas control means is decreasing, the relationship obtained by the learning may deviate from the actual relationship.

Accordingly, in the invention, when the learning means learns the relationship between the intake air amount and the temperature of the exhaust gas control means, the learning is performed after the temperature of the exhaust gas control means is decreased to a temperature corresponding to the intake air amount that is obtained when the learning is performed, namely, after the temperature of the exhaust gas control means is brought to the steady state. It may be determined that the temperature of the exhaust gas control means has been brought to the steady state, when the amount of change in the temperature of the exhaust gas control means per unit time becomes equal to or smaller than a predetermined amount.

The relationship between the intake air amount and the temperature of the exhaust gas control means is learned when the temperature of the exhaust gas control means is in the steady state, and the intake air amount is controlled based on the relationship obtained by the learning such that the temperature of the exhaust gas control means is controlled to the target temperature. Accordingly, when the temperature increasing condition has been satisfied, even if the operation state of the internal combustion engine is changed due to a change in the engine load from a low engine load to a high engine load, the temperature of the exhaust gas control means can be controlled to the target temperature more reliably.

With the exhaust gas control apparatus for an internal combustion engine according to the invention, the temperature of the exhaust gas control means provided in the exhaust passage of the internal combustion engine can be increased to the target temperature in the broader range of the operation state. Accordingly, the exhaust gas control ability of the exhaust gas control means can be recovered more appropriately.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, advantages thereof, and technical and industrial significance of this invention will be better understood by reading the following detailed description of preferred embodiments of the invention, when considered in connection with the accompanying drawings, in which:

FIG. 1 illustrates a view schematically showing an internal combustion engine and an intake/exhaust system thereof according to a first embodiment of the invention;

FIG. 2 illustrates a flowchart showing a filter temperature increasing control routine according to the first embodiment;

FIG. 3 illustrates a time chart showing a relationship among an engine load, fuel supply from a fuel injection valve, a temperature of a filter, and an opening amount of a throttle valve during the filter temperature increasing control;

FIG. 4 illustrates a flowchart showing a control routine that is used when the engine load of the internal combustion engine is changed from an engine load lower than a predetermined engine load to a load equal to or higher than the predetermined engine load during the filter temperature increasing control;

FIG. 5 illustrates a view showing an internal combustion engine and an intake/exhaust system thereof according to a second embodiment of the invention;

FIG. 6 illustrates a graph showing a relationship between an operation state of the internal combustion engine and a method of increasing the temperature of the filter; and

FIG. 7 illustrates a flowchart showing a filter temperature increasing control routine according to the second embodiment.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

In the following description and the accompanying drawings, the present invention will be described in more detail in terms of exemplary embodiments.

First, a first embodiment of the invention will be described. Hereafter, a description will be made concerning the case where the invention is applied to a diesel engine for driving a vehicle. FIG. 1 illustrates a view schematically showing a structure of an internal combustion engine and an intake/exhaust system thereof according to the first embodiment.

An internal combustion engine 1 is a diesel engine for driving a vehicle. An intake passage 4 and an exhaust passage 2 are connected to the internal combustion engine 1. An airflow meter 11 and a throttle valve 8 are provided in the intake passage 4. A particulate filter 3 (hereinafter, simply referred to as a “filter 3”) which traps PM, for example, soot in the exhaust gas is provided in the exhaust passage 2. The filter 3 supports an oxidation catalyst. The filter 3 may support a NOx storage reduction catalyst instead of supporting the oxidation catalyst. Instead of the structure in which the filter 3 supports a catalyst, a structure, in which an oxidation catalyst, or the like is provided in the exhaust passage 2 at a position upstream of the filter 3, may be employed.

A fuel supply valve 5 which supplies fuel in the exhaust gas is provided in the exhaust passage 2 at a position upstream of the filter 3. An exhaust gas temperature sensor 7 which outputs an electric signal corresponding to the temperature of the exhaust gas flowing through the exhaust passage 2 is provided in the exhaust passage 2 at a position downstream of the filter 3.

For the internal combustion engine 1 having the above-mentioned structure, an electronic control unit (ECU) 10 for controlling the internal combustion engine 1 is provided. The ECU 10 controls an operation state of the internal combustion engine 1 based on operation conditions of the internal combustion engine 1 or a request made by a driver. The ECU 10 is electrically connected to various types of sensors such as the airflow meter 11, the exhaust gas temperature sensor 7, a crank position sensor 6 that outputs an electric signal corresponding to a crank angle of the internal combustion engine 1, and an accelerator pedal operation amount sensor 9 that outputs an electric signal corresponding to an accelerator pedal operation amount. The ECU 10 receives signals output from these sensors. The ECU 10 calculates an engine rotational speed of the internal combustion engine 1 based on a value output from the crank position sensor 6, and calculates an engine load of the internal combustion engine 1 based on a value output from the accelerator pedal operation amount sensor 9. The ECU 10 estimates the temperature of the filter 3 based on a value output from the exhaust gas temperature sensor 7. The ECU 10 is electrically connected to the throttle valve 8, the fuel supply valve 5, a fuel injection valve of the internal combustion engine 1, and the like, and therefore the ECU 10 can control these valves.

First, a filter temperature increasing control will be described. If PM is accumulated in the filter 3, the exhaust gas control ability of the filter 3 is decreased. Therefore, in the first embodiment, if the amount of PM accumulated in the filter 3 becomes equal to or larger than a predetermined accumulated amount, the filter temperature increasing control for increasing the temperature of the filter 3 to the target temperature is performed in order to oxidize and remove the PM accumulated in the filter 3, thereby recovering the exhaust gas control ability of the filter 3. The predetermined accumulated amount is an amount that is smaller than the PM accumulated amount at which the temperature of the filter 3 is excessively increased due to the heat generated by oxidation of the PM, and that is set in advance by an experiment or the like. The target temperature is a temperature at which the accumulated PM can be oxidized and removed. The filter temperature increasing control may be performed at predetermined time intervals or at predetermined distance intervals.

Hereafter, the filter temperature increasing control routine according to the first embodiment will be described with reference to a flowchart shown in FIG. 2. The routine is stored in the ECU 10 in advance, and repeatedly performed at predetermined time intervals while the internal combustion engine 1 is operated.

In this routine, the ECU 10 determines in step S101 whether an execution condition of the filter temperature increasing control has been satisfied. When an affirmative determination is made in step S101, the ECU 10 performs step S102. On the other hand, when a negative determination is made in step S101, the ECU 10 ends the routine.

In step S102, the ECU 10 determines whether the present engine load of the internal combustion engine 1 is equal to or higher than a predetermined engine load.

The predetermined engine load is a threshold value of the engine load at which it can be determined that the temperature of the exhaust gas flowing through the exhaust passage 2 located upstream of the filter 3 has become so high that the fuel supplied from the fuel supply valve 5 is burned in the exhaust passage 2. When the engine load of the internal combustion engine 1 is at substantially the same value, as the engine rotational speed of the internal combustion engine 1 increases, the amount of fuel injected by the main fuel injection is increased. Therefore, the temperature in the cylinder 2 and the exhaust gas temperature may be increased. Accordingly, in the first embodiment, as the present engine rotational speed of the internal combustion engine 1 becomes higher, the predetermined engine load is set to a lower value. In the first embodiment, a map defining the relationship between the engine rotational speed of the internal combustion engine 1 and the predetermined engine load may be stored in the ECU 10 in advance. When an affirmative determination is made in step S102, the ECU 10 performs step S103. On the other hand, when a negative determination is made in step S102, the ECU 10 performs step S104.

In step S104, the ECU 10 causes the fuel supply valve 5 to supply fuel, thereby supplying fuel to the filter 3, and controls the amount of fuel supplied from the fuel supply valve 5. Thus, when the present temperature of the filter 3 is lower than the target temperature, the ECU 10 increases the temperature of the filter 3 to the target temperature. On the other hand, when the present temperature of the filter 3 is equal to the target temperature, the ECU 10 maintains the temperature of the filter 3 at the target temperature. Namely, fuel is supplied from the fuel supply valve 5, whereby the fuel is supplied to the oxidation catalyst supported by the filter 3. Then, the temperature of the filter 3 is increased by the heat generated due to oxidation of the fuel that occurs in the oxidation catalyst. When fuel is supplied from the fuel supply valve 5 in step S104, the opening amount of the throttle valve 8 is controlled to an opening amount that is set based on the engine rotational speed and the engine load of the internal combustion engine 1. At this time, the opening amount of the throttle valve 8 may be different from an opening amount that is used when the engine rotational speed of the internal combustion engine 1 is at substantially the same value and the engine load of the internal combustion engine 1 is at substantially the same value in the normal operation state. After the temperature of the filter 3 is increased to the target temperature in step S104, the ECU 10 ends the routine.

On the other hand, in step S103, the ECU 10 prohibits fuel supply from the fuel supply valve 5 and decreases the opening amount of the throttle valve 8, thereby decreasing the intake air amount and increasing the temperature of the filter 3.

As described above, if fuel is supplied from the fuel supply valve 5 when the engine load is equal to or higher than the predetermined engine load, the fuel is burned in the exhaust passage 2. Therefore, in the case where fuel is supplied from the fuel supply valve 5, it may be difficult to supply the oxidation catalyst supported by the filter 3 with the sufficient amount of fuel required for increasing the temperature of the filter 3 to the target temperature. Accordingly, in step S103, the opening amount of the throttle valve 8 is controlled, namely, the intake air amount is controlled. Thus, when the present temperature of the filter 3 is lower than the target temperature, the temperature of the filter 3 is increased to the target temperature. On the other hand, when the present temperature of the filter 3 is equal to the target temperature, the temperature of the filter 3 is maintained at the target temperature.

In this case, the exhaust gas temperature is increased by decreasing the intake air amount, and the temperature of the filter 3 is increased with an increase in exhaust gas temperature. In this case, the relationship between the opening amount of the throttle valve 8 and the temperature of the filter 3 in the state where fuel supply from the fuel supply valve 5 is not performed is learned before the opening amount of the throttle valve 8 is changed, and the opening amount of the throttle valve 8 is controlled based on the relationship obtained by the learning. After the temperature of the filter 3 is increased to the target temperature in step S103, the ECU 10 ends the routine.

According to the control routine described so far, the temperature of the filter 3 can be increased to the target temperature regardless of the engine load of the internal combustion engine 1. Namely, the temperature of the filter 3 can be increased to the target temperature in a broader range of operation state. It is, therefore, possible to oxidize and remove the PM accumulated in the filter 3 in the broader range of operation state. When the engine load of the internal combustion engine 1 is equal to or higher than the predetermined engine load, prohibition of fuel supply from the fuel supply valve 5 makes it possible to suppress an excessive increase in the exhaust gas temperature caused due to burning of the fuel in the exhaust passage 2. According to the first embodiment, the exhaust gas control ability of the filter 3 can be recovered appropriately.

In the above-mentioned control routine, when fuel is supplied to the filter 3 in order to increase the temperature of the filter 3, the fuel is supplied from the fuel supply valve 5 to the exhaust gas. However, fuel may be supplied to the filter 3 from a position upstream of the filter 3 by performing sub-fuel injection in the cylinder of the internal combustion engine 1 during the power stroke or the exhaust stroke after the main fuel injection is performed, instead of by performing fuel supply from the fuel supply valve 5.

The control routine may be applied to the case where the filter 3 supports a NOx storage reduction catalyst (hereinafter, referred to as a “NOx catalyst”), and SOx stored in the NOx catalyst is reduced. In this case, the temperature of the filter may start to be increased when the amount of SOx stored in the NOx catalyst becomes equal to or larger than a predetermined storage amount. Also, the target temperature is a temperature at which the SOx stored in the NOx catalyst can be reduced.

The above-mentioned control routine may be applied to the case where the NOx catalyst is provided instead of the filter 3, and the temperature of the NOx catalyst is increased.

Next, the learning of the relationship between the opening amount of the throttle valve and the temperature of the filter will be described. Namely, a description will be made concerning the control that is performed when the engine load of the internal combustion engine 1 is changed from an engine load lower than the predetermined engine load to an engine load equal to or higher than the predetermined engine load during the filter temperature increasing control, with reference to FIG. 3. FIG. 3 illustrates a time chart showing a relationship among the engine load, the fuel supply from the fuel supply valve 5, the temperature of the filter 3, and the opening amount of the throttle valve 8 during the filter temperature increasing control. The chained line in the region showing the engine load indicates the predetermined engine load, and the chained line in the region showing the temperature of the filter indicates the target temperature.

As described above, when the engine load of the internal combustion engine 1 is lower than the predetermined engine load during the filter temperature increasing control, fuel supply from the fuel supply valve 5 is performed. If the engine load of the internal combustion engine 1 is changed to an engine load equal to or higher than the predetermined engine load, fuel supply from the fuel supply valve 5 is stopped upon completion of the change in the engine load, as shown in FIG. 3. As a result, the temperature of the filter 3, which has been maintained at the target temperature, starts to decrease.

In the first embodiment, when the engine load of the internal combustion engine 1 is equal to or higher than the predetermined engine load during the filter temperature increasing control, the temperature of the filter 3 is increased to the target temperature by controlling the opening amount of the throttle valve 8. In this case, the ECU 10 learns the relationship between the temperature of the filter 3 and the opening amount of the throttle valve 8 in the state where fuel supply from the fuel supply valve 5 is not performed. The opening amount of the throttle valve 8 is controlled based on the relationship obtained by the learning. However, if the ECU 10 learns the relationship between the temperature of the filter 3 and the opening amount of the throttle valve 8 while the temperature of the filter 3 is decreasing, the ECU 10 erroneously learns that the relationship using a temperature that is higher than the temperature of the filter 3 actually corresponding to the opening amount of the throttle valve 8. Then, if ECU 10 controls the opening amount of the throttle valve 8 in order to increase the temperature of the filter 3 to the target temperature based on the relationship between the temperature of the filter 3 and the opening amount of the throttle valve 8 that is obtained by learning performed in the above-mentioned manner, the actual temperature of the filter 3 deviates from the target temperature.

Accordingly, in the first embodiment, after the fuel supply from the fuel supply valve 5 is stopped and the temperature of the filter 3 is brought into the steady state (namely, at time “a” in FIG. 3), the ECU 10 learns the relationship between the temperature of the filter 3 and the opening amount of the throttle valve 8. Then, the opening amount of the throttle valve 8 is controlled such that the temperature of the filter 3 is increased to the target temperature based on the relationship between the temperature of the filter 3 and the opening amount of the throttle valve 8 that is obtained by learning performed at time “a” shown in FIG. 3.

Next, the control routine that is used when the engine load is changed during the filter temperature increasing control will be described. Hereafter, a description will be made concerning the control routine that is used when the engine load of the internal combustion engine 1 is changed from an engine load lower than the predetermined engine load to an engine load equal to or higher than the predetermined engine load during the filter temperature increasing control, with reference to a flowchart in FIG. 4. The routine is stored in the ECU 10 in advance, and repeatedly performed at predetermined intervals during the filter temperature increasing control.

In the routine, the ECU 10 determines in step S201 whether the engine load of the internal combustion engine 1 has been changed from an engine load lower than the predetermined engine load to an engine load equal to or higher than the predetermined engine load. When an affirmative determination is made in step S201, the ECU 10 performs step S202. On the other hand, when a negative determination is made in step S201, the ECU 10 ends the routine.

In step S202, the ECU 10 stops the fuel supply from the fuel supply valve 5.

Next, the ECU 10 determines in step S203 whether the temperature of the filter 3 is in the steady state. An affirmative determination is made in step S203 if the amount of change in the temperature of the filter 3 per unit time becomes equal to or smaller than a predetermined amount. When an affirmative determination is made in step S203, the ECU 10 performs step S204. On the other hand, when a negative determination is made in step S203, the ECU 10 ends the routine. When a negative determination is made in step S203, the ECU 10 may repeatedly perform step S203.

In step S204, the ECU 10 learns the relationship between the opening amount of the throttle valve 8 and the temperature of the filter 3.

Next, in step S205, the ECU 10 controls the opening amount of the throttle valve 8 based on the relationship between the opening amount of the throttle valve 8 and the temperature of the filter 3 that is obtained by learning performed in step S204, so as to increase the temperature of the filter 3 to the target temperature, after which the ECU 10 ends the routine.

According to the control routine described so far, the relationship between the opening amount of the throttle valve 8 and the temperature of the filter 3 is learned when the temperature of the filter 3 is in the steady state, and the opening amount of the throttle valve 8 is controlled based on the relationship obtained by the learning such that the temperature of the filter 3 becomes the target temperature. Thus, even when the operation state of the internal combustion engine 1 is changed due to a change in the engine load from a low engine load to a high engine load while the temperature increase condition is satisfied, the temperature of the filter 3 can be controlled to the target temperature more reliably.

In the first embodiment, when an exhaust throttle valve is provided in the exhaust passage 2, control of the opening amount of the exhaust throttle valve may be performed along with the control of the opening amount of the throttle valve 8.

Next, a second embodiment of the invention will be described. FIG. 5 illustrates a view schematically showing an internal combustion engine and an intake/exhaust system thereof according to the second embodiment. As shown in FIG. 5, the schematic structure of the internal combustion engine and the intake/exhaust system thereof according to the second embodiment is the same as that according to the first embodiment except that the fuel Supply valve 5 is not provided in the second embodiment.

First, the filter temperature increasing control will be described. In the second embodiment, when the temperature of the filter 3 is increased by supplying fuel to the filter 3 (the oxidation catalyst supported by the filter 3), sub-fuel injection is performed in the cylinder of the internal combustion engine 1 during the power stroke or the exhaust stroke after the main fuel injection is performed, whereby fuel is supplied to the filter 3 from a position upstream of the filter 3.

When fuel is supplied to the filter 3 by the sub-fuel injection, as the engine rotational speed of the internal combustion engine 1 becomes higher, the time required for one combustion cycle becomes shorter. Therefore, the number of times of the sub-fuel injection that can be performed during one combustion cycle decreases. Accordingly, in the region where the internal combustion engine 1 is operated at a high engine rotational speed, it may be difficult to supply a sufficient amount of fuel required for increasing the temperature of the filter 3 to the target temperature by performing the sub-fuel injection.

Accordingly, in the second embodiment, as shown in FIG. 6, when the filter temperature increasing control is performed while the operation state of the internal combustion engine 1 is in a region (A), namely, when the engine load of the internal combustion engine 1 is lower than the predetermined engine load and the engine rotational speed of the internal combustion engine 1 is lower than the predetermined engine rotational speed, the temperature of the filter 3 is increased to the target temperature by performing the sub-fuel injection and controlling the amount of fuel injected by the sub-fuel injection. When the filter temperature increasing control is performed while the operation state of the internal combustion engine 1 is in a region (B), namely, when the engine load of the internal combustion engine 1 is equal to or higher than the predetermined engine load or the engine rotational speed of the internal combustion engine 1 is equal to or higher than the predetermined engine rotational speed, the sub-fuel injection is prohibited, and the temperature of the filter 3 is increased to the target temperature by controlling the opening amount of the throttle valve 8.

FIG. 6 illustrates a graph showing the relationship between the operation state of the internal combustion engine 1 and a method of increasing the temperature of the filter 3. In FIG. 6, the vertical axis indicates the engine load, and the horizontal axis indicates the engine rotational speed. The dashed line indicates the boundary between the region (A) and the region (B). The solid line indicates the maximum engine load corresponding to the engine rotational speed.

The boundary between the region (A) and the region (B) is set based on the predetermined engine load and the predetermined engine rotational speed. The predetermined engine load is the same as the predetermined engine load according to the first embodiment. As the engine rotational speed becomes higher, the predetermined engine load is set to a lower value. The predetermined engine rotational speed is the engine rotational speed that is set in advance. The predetermined engine rotational speed is the threshold value of the engine rotational speed at which it becomes difficult to perform the sub-fuel injection number of times that makes it possible to supply the sufficient amount of fuel required for increasing the temperature of the filter 3 to the target temperature.

Next, the filter temperature increasing control routine according to the second embodiment will be described with reference to a flowchart shown in FIG. 7. The routine is the same as the filter temperature increasing control routine shown in FIG. 2 except that step S304 is performed instead of step S104, and step S303 is added. Therefore, only step S303 and step S304 will be described, and the other steps will not be described here. As in the case of the routine shown in FIG. 2, the routine according to the second embodiment is stored in the ECU 10 in advance, and repeatedly performed at predetermined time intervals during the operation of the internal combustion engine.

In the routine, when a negative determination is made in step S102, the ECU 10 performs step S303.

The ECU 10 determines in step S303 whether the engine rotational speed of the internal combustion engine 1 is equal to or higher than the predetermined engine rotational speed. When an affirmative determination is made in step S303, the ECU 10 performs step S103. On the other hand, when a negative determination is made in step S303, the ECU 10 performs step S304.

In step S304, the ECU 10 performs the sub-fuel injection in the cylinder of the internal combustion engine 1, and controls the amount of fuel injected by the sub-fuel injection. Thus, when the present temperature of the filter 3 is lower than the target temperature, the temperature of the filter 3 is increased to the target temperature. When the present temperature of the filter 3 is equal to the target temperature, the temperature of the filter 3 is maintained at the target temperature. When the sub-fuel injection is performed in step S304, the opening amount of the throttle valve 8 is controlled to the opening amount that is set based on the engine load of the internal combustion engine 1.

According to the control routine described so far, even if the engine rotational speed of the internal combustion engine 1 is high when fuel is supplied to the filter 3 by the sub-fuel injection, the temperature of the filter 3 can be increased to the target temperature more reliably. Namely, the temperature of the filter 3 can be increased to the target temperature in a broader range of the operation state. Accordingly, the PM accumulated in the filter 3 can be oxidized and removed in the broader range of operation state.

In the second embodiment, when the operation state of the internal combustion engine 1 is shifted from the region (A) to the region (B) shown in FIG. 6 during the filter temperature increasing control, the sub-fuel injection is stopped upon completion of the shifting. Then, the ECU 10 learns the relationship between the temperature of the filter 3 and the opening amount of the throttle valve 8 after the sub-fuel injection is stopped and the temperature of the filter 3 is brought to the steady state. Then, the ECU 10 controls the opening amount of the throttle valve 8 based on the relationship obtained by the learning so as to increase the temperature of the filter 3 to the target temperature.

Thus, even when the operation state of the internal combustion engine 1 is changed during the filter temperature increasing control, the temperature of the filter 3 can be controlled to the target temperature more reliably.

As in the case of the filter temperature control routine according to the first embodiment, the control routine according to the second embodiment can be applied to the case where SOx stored in the NOx catalyst is reduced when the NOx catalyst is supported by the filter 3 or the NOx catalyst is provided instead of the filter 3. 

1. An exhaust gas control apparatus for an internal combustion engine, comprising: an exhaust gas control device which is provided in an exhaust passage of an internal combustion engine, and which has oxidation function; fuel supply device for supplying fuel to the exhaust gas control device from a position upstream of the exhaust gas control device; and intake air amount control device for controlling an amount of air taken in the internal combustion engine, wherein, when a temperature increasing condition for increasing a temperature of the exhaust gas control device to a target temperature in order to recover an exhaust gas control ability of the exhaust gas control device has been satisfied, if an engine load of the internal combustion engine is lower than a predetermined engine load, the temperature of the exhaust gas control device is increased to or maintained at the target temperature by controlling an amount of fuel supplied from the fuel supply device to the exhaust gas control device, and the intake air amount control device controls the amount of air taken in the internal combustion engine to an intake air amount that is set based on an engine rotational speed of the internal combustion engine and the engine load of the internal combustion engine; and when the temperature increasing condition has been satisfied, if the engine load of the internal combustion engine is equal to or higher than the predetermined engine load, fuel supply from the fuel supply device is prohibited, and the intake air amount control device controls the amount of air taken in the internal combustion engine, whereby the temperature of the exhaust gas control device is increased to or maintained at the target temperature.
 2. The exhaust gas control apparatus for an internal combustion engine according to claim 1, wherein the exhaust gas control device includes at least one of a filter which supports an oxidation catalyst, a filter which supports a NOx storage reduction catalyst, and a NOx storage reduction catalyst.
 3. The exhaust gas control apparatus for an internal combustion engine according to claim 1, wherein the fuel supply device performs sub-fuel injection in a cylinder of the internal combustion engine during a power stroke or an exhaust stroke after main fuel injection is performed, thereby supplying fuel to the exhaust gas control device from a position upstream of the exhaust gas control device.
 4. The exhaust gas control apparatus for an internal combustion engine according to claim 3, wherein even in a case where the engine load of the internal combustion engine is lower than the predetermined engine load when the temperature increasing condition has been satisfied, if the engine rotational speed of the internal combustion engine is equal to or higher than a predetermined engine rotational speed, the fuel supply from the fuel supply device is prohibited, and the intake air amount control device controls the amount of air taken in the internal combustion engine, whereby the temperature of the exhaust gas control device is increased to or maintained at the target temperature.
 5. The exhaust gas control apparatus for an internal combustion engine according to claim 1, wherein the fuel supply device supplies fuel by using a fuel supply valve that is provided in the exhaust passage at a position upstream of the exhaust gas control device, thereby supplying the fuel to the exhaust gas control device.
 6. The exhaust gas control apparatus for an internal combustion engine according to claim 1, wherein, as the engine rotational speed of the internal combustion engine that is obtained when the temperature increasing condition has been satisfied becomes higher, the predetermined engine load is set to a lower value.
 7. The exhaust gas control apparatus for an internal combustion engine according to claim 1, wherein there is further provided learning device for learning a relationship between the amount of air taken in the internal combustion engine and the temperature of the exhaust gas control device in a state where the fuel supply from the fuel supply device is not performed, wherein in a case where the temperature increasing condition has been satisfied, when an operation state of the internal combustion engine is shifted from an operation region where the amount of fuel supplied from the fuel supply device to the exhaust gas control device is controlled, whereby the temperature of the exhaust gas control device is increased to the target temperature, to an operation region where the intake air amount control device controls the amount of air taken in the internal combustion engine, whereby the temperature of the exhaust gas control device is increased to the target temperature, the relationship between the amount of air taken in the internal combustion engine and the temperature of the exhaust gas control device is learned by the learning device after the fuel supply from the fuel supply device is stopped and the temperature of the exhaust gas control device is brought to a steady state; and the amount of air taken in the internal combustion engine is controlled by the intake air amount control device based on the relationship between the amount of air taken in the internal combustion engine and the temperature of the exhaust gas control service that is obtained by learning performed by the learning device such that the temperature of the exhaust gas control device is controlled to the target temperature. 